…Evolution & Bisexual Monkeys

Parts 5-7: What's that about?

Part 5: A Ridiculous Story

A couple nights ago, Wendy and I were sitting on our balcony having a cocktail and talking about this essay. Let me tell you a ridiculous story I made up just to illustrate a point….

We all know that the “typical” male and female body types differ because of differences in bone structure, muscle mass, and fat distribution. Here’s the evolutionary reason why. When humans were evolving, males spent their time chasing down and butchering big game, and carrying it back to home base. Women spent their time tending to children and preparing fruits and vegetables. These differences in physical activity led to large differences in energy expenditure, and men were more tired and physically drained at the end of the day than were women. Sleeping on the hard ground did not provide the comfort necessary for men to rest and recuperate sufficiently. So, women evolved softer, curvier bodies so that men could cuddle up and lean on them at night providing them with much-needed restorative sleep. Those women who were softest and most cuddly had partners that were best rested and could more effectively hunt during the day. Consequently, those couples had more food resources and could successfully raise more children. So, the genes for curvy, cuddly female bodies were passed on to the next generation.

Pretty ridiculous story, isn’t it?

Yet, when I tell stories like this to make a point, most people (especially my students) think they’re as plausible as any other evolutionary story they’ve heard… because, basically, it’s the same kind of story they’ve been listening to since they were kids. And, as I said last time, it’s the same kind of story that’s taught in biology classes about the crossbill finch’s unusual beak. But, more importantly, it’s the same kind of story that shows up repeatedly in popular articles about human behavior (here, here, here, here, here).

Here’s what’s wrong with the story.

First, the story assumes there’s an evolutionary — that is, reproductively advantageous — reason that male and female bodies differ and the reason is the one that happened to pop into the storyteller’s head. The first part of this assumption may be true, although it’s an empirical question that needs supporting evidence. However, the second part of the assumption is a fantasy made up by the storyteller, and it’s no more plausible than any other fantasy.

Second, the story doesn’t acknowledge that both male and female bodies vary dramatically within sexes. Some people are soft and curvy (including some men), other people are lean and muscular (including some women). The story only considers the evolution of one type of female body — the one in which the storyteller is interested — and only one supposed evolutionary advantage of that body type… providing some guy with a comfortable night’s sleep.

The third thing that’s wrong is that a trait cannot evolve to solve a problem. Biology doesn’t care if your partner’s exhausted when he comes back to base camp. In fact, biology doesn’t care if he comes back at all. If he makes it back, good for him. But nobody’s going to evolve into a human pillow because he’s grumpy and tired. He’ll just have to deal with it.

Fourth — yes, there’s more — this kind of story assumes that there’s a straight-line evolutionary path from early hominids (the family that includes some great apes and humans) to the contemporary people at the end of the story. This kind of just-so story rarely — if ever — includes any details about the probable evolutionary processes that gave rise to the trait in question, especially when the story is about people. The reason is because the storyteller usually doesn’t have a clue.

Consider this. There have been about a dozen different species of humans on the planet (maybe more, depending on whom you read). At what point in the human evolutionary tree would the “curvy & cuddly” trait have first appeared, and under what social and environmental conditions were those humans living when it did? I can’t claim the trait has any reproductive advantage if I don’t have answers to these questions.

Also note that some modern humans are hybrids of at least two other human subspecies. Could the trait just be a serendipitous byproduct of a hybridization event? Or, perhaps, it was just the outcome of some random genetic mutation with no reproductive implications at all (like the crossbill’s beak).

Whenever I read a just-so story about the evolution of some human trait, it generally begins with a hypothetical prehistoric person — usually a caricature of a “caveman” struggling to survive on the “savanna” — and ends with a contemporary person who’s inherited the trait because he’s a direct descendent of the prehistoric guy. It’s a bit naïve.

Of course, evolution happens. But, it’s not a process that creates specific traits to solve specific problems, and it doesn’t create stereotypical, idealized organisms. And, most importantly, you just can't make up a random story about evolution to suit your needs (unless you just want to sound ridiculous).

Evolution by natural selection creates potential breeding populations of organisms that vary (sometimes dramatically) in their individual characteristics. As I noted above, some people are soft and curvy, some people are lean and hard. Some people are very short, some people are very tall. Some people are very big, some people are very small. Evolution didn’t create the perfectly adapted human. It created a population of very different individuals who can breed with each other. If enough people are sufficiently functional to have kids that live, our species survives another generation. That’s the only criterion biology has.

Part 6: Really, Bisexual Monkeys?

If you think that everything in nature has a purpose, or that every physical trait or behavior confers some reproductive advantage, or that evolution creates stereotypical, idealized organisms, you’ve got two problems. The first one is that you have to make up a just-so story for everything that you see. The second problem is that if someone discovers something that doesn’t seem to have an evolutionary purpose, or doesn’t fit your stereotype of what an organism should be, you’ll be inclined to call them a liar. That’s what happened to Linda Wolfe, the former Chair of Anthropology at East Carolina University (Greenville, North Carolina).

In the 1970s, Linda was a graduate student studying Japanese macaque monkeys. Surprisingly, she discovered that female macaques were having sex with other females. Sometimes the encounters were brief. However, sometimes females formed exclusive couples that stayed together for weeks, cuddling, grooming, sleeping together, and engaging in frequent bouts of intense sexual activity that culminated in mutual orgasms. By the way, these females also had sex with male macaques.    

Linda was excited about her discovery. The scientific establishment, not so much. She was accused of having “kinky” interests, doctoring her photographs and faking her data. Some scientists claimed that female monkeys were mistakenly hooking up with other females because “they didn’t know what they were doing.”

Why this resistance to Wolfe’s findings? What’s the difference if macaque monkeys are bisexual? Well, there were two problems. First, sex between females doesn’t have an obvious biological purpose. It’s inconsistent with the belief that everything in nature is purposeful and linked to successful reproduction. The second problem is that it’s inconsistent with the idea that evolution shapes stereotypical organisms with just the “right” set of characteristics and behaviors. In Wolfe’s words, “People wanted to believe that only weirdo humans engaged in this [homosexual] behavior.”

Just to be sure that I correctly understood the resistance to her work, I emailed Wolfe last week. I asked her if she thought the pushback stemmed from the erroneous idea that everything in nature has a “purpose.” She replied:

Yes, you are correct that much of the resistance to the idea of same-sex interactions which are found in many species… is the purpose argument.  Not all behavior has to have a purpose… as Paul [Vasey] has said, some of it is just for fun.  Male macaques masturbate a lot during the breeding season and no man every worried about the purpose of all the sperm lost… because they… imagine that male macaques have the same sex pleasure as human males.  It was only female macaques that should not have sexual pleasure because it would serve no purpose.

Paul Vasey also thinks that because “female homosexual… activity is confined to a restricted geographic region within Japanese macaque habitat” that the behavior may “be associated with genetically distinct… macaque populations.” If true, this means that macaque female-female sexual behavior is a result of genetic changes that serve no evolutionary or reproductive purpose… like the crossbill’s strange beak, or a turtle’s second head.

So, it seems that macaque bisexuality is just one of those interesting biological phenomena that make the natural world so fascinating.

Part 7: Can Monkeys Be Non-binary?

Takasakiyama Monkey Park in Japan is home to about 1500 free ranging macaque monkeys that have formed two separate troops of about 700-800 individuals. The troops spend most of their time in the forested slopes around Mount Takasaki. However, every morning and afternoon they take turns coming down into the park to be fed by the wardens.

By the way, macaques have very strong dominance hierarchies among both the males and the females.

Last year, Yakei, a 9-year-old female, fought her way up to the dominant, or Alpha female position in one of the troops. To do this, she had to prove herself tougher than all the other females which included beating up her own mother. 

However, that win wasn’t enough for Yakei. Just a couple months later, she decided to go after the top male in the troop, a rare event in the monkey world. Yakei challenged and beat up Sanchu, an Alpha male who had been the head of the troop for five years. This made Yakei the first ever Alpha female monkey boss of an entire troop in the 70-year history of the Monkey Park.

Since her ascent to the top of the troop’s hierarchy, Yakei has been flaunting her power by shaking trees and walking around with her tail up which are dominance gestures seldom displayed by female macaques.

So, considering female macaque bisexuality and Yakei’s atypical aggressive behavior, some people might be tempted to say that she’s transsexual.

Of course, I would never suggest that. I learned from Happy the elephant that you can’t assume the inner thoughts of another organism… especially one that doesn’t have the cognitive or language abilities to discuss abstract ideas like gender… You know, like monkeys and children.

I also know that evolution doesn’t create idealized animals — or people — that fit my stereotypes of how males and females should behave.

Believing that a primate — whether it’s a macaque or a human — is born in the wrong body because their behaviors don’t match my stereotypes would be as ridiculous as the story I told at the beginning of this essay.

 I try to avoid being ridiculous whenever I can. Usually, I succeed.